A photodiode is typically mounted and sealed into a windowed housing, such as a TO-8 metal can or other such containers. When mounting the canned photodiode on to a printed wiring board (PWB) or other carrier, it is generally difficult to locate the diode's active area in specific X, Y, and Z axes within a certain tolerance range. In addition, once the photodiode is hermetically sealed inside the can so that the active area is underneath the glass window inside, it is inaccessible to contact or probing of any kind. Thus, direct manipulation of the photodiode sealed within a mounted can to locate the diode's active area in the specific X, Y and Z axes is precluded. Such direct manipulation might damage the photodiode, anyway.
As such, locating the diode's active area in the specific X, Y, and Z axes within a certain tolerance range must be performed during the mounting process. Currently, a photodiode is typically soldered directly to a printed wiring board with no control of the X, Y and Z locations of its active area. For many applications, this non-controlled mounting process produces sufficient optical alignment for the photodiode to carry out its intended function, such as detection of infrared light (IR). However, some photodiode applications require a more stringent tolerance range on the X, Y, and Z location of the active area of the photodiode. Conventional fabrication processes are simply not sufficiently controlled to meet these tighter tolerance ranges required for functionality of the end product.
What is needed, therefore, are techniques for locating the active area of a photodiode in the X, Y, and Z optical axes within a certain tolerance range.
More particularly, in infrared missile guidance systems it is often necessary to be able to collect as much light as possible from reflections from targets illuminated by laser target designators. In order to detect the returns from the laser target designator, typically an array of photodetectors is arranged to detect the reflections, with each of the photodetectors provided with an imaging or relay lens adjacent the photodetector so that the reflected light is focused onto the active area of the photodiode.
The basic problem is to locate an avalanche photodiode on a printed circuit board exactly at the focus of the lens that focuses the light onto the avalanche photodiode. This distance needs to be controlled so that the maximum amount of light falls on the diode. If the diode is too far away, the cone that is formed by the lens spreads. If the photodiode is too close to the lens, the light is too scattered.
In the past, avalanche photodiodes being within the hermetically sealed can have been positioned when the diode is turned on and the position of the can moved until a maximum in the output of the diode is reached. This constitutes an active tuning system.
The problem, however, is how to tune or locate the avalanche photodiode by passive techniques. Thus, prior to manufacture it is important to be able to accurately position the photodiode at the focus of its associated lens prior to activating the photodiode. Because the photodiode surface is underneath a piece of glass and in a sealed can, one cannot access it physically. Thus, its position up and down and side to side inside the can varies. Since one cannot physically touch the photodiode surface, there are no mechanical means by which the photodiode surface can be placed exactly at the focus of the associated lens.
As a result, the tolerance by which the photodiode active surface is located relative to the fixed outer dimensions of the can do not provide the required accuracy for photodiode active area placement especially in laser target designation applications.
In one typical missile guidance or seeker applications for infrared return detection, the tolerance for the surface of the active area of the photodiode being at the focus of the lens is on the order of plus or minus 0.003 inches.
In short, a manufacturing technique is required to be able to locate the avalanche photodiode can in an associated carrier such that by manipulation of the can one can place the active photodiode surface at a precise distance from a lens corresponding to the focal point of the lens.